Such methods and plants are used for instance for the reduction of ilmenite (x*TiO2y*FeOz*Fe2O3). For this purpose, ilmenite is treated for example in rotary kilns (for instance the SLRN method) with suitable carbons at temperatures of between 850 and 1200° C. Depending on the type of treatment, the reduction of the iron may be undertaken in a further processing stage to FeO or to metallic iron. For example, a high degree of metallization of the iron of up to 97% in the reduced ilmenite is the target for the so-called Becher method.
However, the metallization of the iron at such high temperatures of 1060 to approximately 1200° C. leads to the formation of undesired complex compounds, known as M3O5 phases, in the ilmenite grain, the letter “M” generally standing for metal, such as for example Ti2MgO5, Ti2MnO5 or Ti2FeO5. Since these compounds are for example neither soluble in sulphuric acid nor in hydrochloric acid, they cannot be dissolved, or only with difficulty, in the hydrometallurgical process stages following the reduction. This has the consequence that, apart from the desired TiO2, undesired impurities remain in the solid product, known as “synthetic rutile”. The production of these undesired compounds is in this case dependent on the temperature and the retention time of the ilmenite in the reduction zone, which in a rotary kiln for example is four to five hours. For many iron-rich ilmenites, the wet-metallurgical enrichment stage is indispensable to produce an end product with good selling properties (synthetic rutile).
Furthermore, methods and plants as mentioned above are also used for the magnetic roasting of ilmenite. For this purpose, previously ilmenite has been subjected to dust-free, for example pre-heated, air through a tuyere bottom (gas distributor) in a circulating fluidized bed. In this case it is found to be disadvantageous that dust-laden gas cannot be used for the fluidizing of the solids. A further disadvantage of this known method is that the combustion profile is unfavourable and, furthermore, there is no utilization of the waste heat of the solids. In part-load operation, there is also the risk that, in spite of the sophisticated mechanical feature of the tuyere bottom, fine-grained solids can undesirably fall through it. The retention time for the solids of 20 to 30 minutes, necessary for process engineering reasons, can be achieved only with a very high pressure loss in the reactor, which in turn leads to undesired pulsations of the fluidized bed. Therefore, these plants must be designed for high dynamic loads in order to be able to withstand the forces occurring during operation.
Reactors with either a stationary fluidized bed or a circulating fluidized bed are generally known for the heat treatment of solids. However, the utilization of the reducing agent and the energy utilization achieved when using a stationary fluidized bed are in need of improvement. One reason for this is that the mass and heat transfer is moderate on account of the comparatively low degree of fluidization. Therefore, an internal combustion that occurs during the magnetic roasting can also only be controlled with difficulty. Furthermore, pre-heating of the solids or cooling of the product can hardly be integrated in a suspension heat exchanger or a fluidized bed cooler, because dust-laden gases are rather not admitted to the fluidizing nozzles of the stationary fluidized bed. Due to the high degree of fluidization, circulating fluidized beds on the other hand have better conditions for mass and heat transfer and allow the integration of a suspension heat exchanger or product cooling, but are restricted in terms of their solids retention time due to the relatively high degree of fluidization.